Avermectin compounds have been known for some time as potent anthelmintic agents and substantial research has been carried out preparing various substituted variations of such compounds. Some of the avermectin compounds have become commercially available as potent broad-spectrum anthelmintic and antiparasitic agents in animal health and agriculture. See US 4310519 to Albers-Schonberg et al and 4199569 to Chabala et al. Applicants are not aware of any avermectin compounds where the 23-ring carbon has been replaced by a heteroatom.